The effects of filtering RF source phase noise by a low noise, high quality factor actively modelocked laser on the laser’s absolute and relative phase noise

Franklyn Quinlan,Peter Delfyett,Sangyoun Gee,Sarper Ozharar

doi:10.1364/oe.14.005346

Abstract

The phase noise of two low noise, high quality factor actively modelocked lasers is investigated. It is found that increasing the quality factor of a laser can increase the phase noise relative to the RF source used to modelock the laser, even though the absolute noise of the laser is decreased. The filtering of phase noise from the modelocking source that causes both the increase in relative noise and the decrease in absolute noise is exploited to reveal phase noise information otherwise obscured in a high quality factor laser.

Highlights

Low noise modelocked lasers have found use in a variety of systems such as optical clock distribution, arbitrary waveform synthesis [1], low noise synthesis of microwave signals [2], and analog-to-digital converters [3]
When the active modelocking is non-regenerative, the filtering effect would result in noise present in the master clock not being present in the laser output, and the relative jitter may be dominated by the noise of the master clock
Beginning at frequencies below 100 kHz, there is a significant departure of the absolute phase noise of the semiconductor laser compared to that of the RF source indicating a filtering of the RF source noise when it is transferred onto the timing jitter

Summary

Introduction

Low noise modelocked lasers have found use in a variety of systems such as optical clock distribution, arbitrary waveform synthesis [1], low noise synthesis of microwave signals [2], and analog-to-digital converters [3]. The well known complication of supermode noise spurs encountered in harmonic modelocking has been thoroughly addressed, and significant improvements in the relative jitter have been achieved [8] Another advantage of increasing the quality factor is the reduction of the linewidth of the optical mode, important in coherent communication systems [16]. There is, another complication that could arise associated with a high quality factor laser that would increase the relative jitter between an actively modelocked laser and the master clock. The filtering effect was exploited to reveal the uncorrelated noise contributed as well as parts of the absolute noise spectrum of one of the lasers

Phase noise and phase noise measurement schemes

Devices under test

Frequency discriminator and relative noise measurements

Lasers in series configuration

Discussion and conclusion